Do you know any good tricks for erecting a gin pole or shear legs without a crane?
jsw
I will try to explain it.
The gin poles we used in the oilfield were miniature jib cranes. They were triangular in form, tapering from the base to the tip, where the single pulley sheave was. There was no compound lifting sheaves, just a push/pull that ran to a drawworks cathead capstan. As with anything, if all the pressure is compression, and not sideloading, they will be incredibly strong, and overkill.
The base of the gin poles we used was just a right angle of heavy flat bar that rested on the bolts of the highest point of where bolts were used to fasten two sections of outer legs together. A chain was used to hold the base on the bolts, and the inward angle of the outer legs, plus a slight cant in the base of the gin pole to the tip made it tip towards the inside of the four sided hole at the top of the derrick, as does the jib section of a common crane. When the element came loose, it was in the position to be lowered through the middle of the square derrick. It is not, however, adjustable, as a crane jib is, being held in place by the chain. In your case with a triangular structure, you could make a bolt on heavy flat bar to secure it on the mast. Or you might have to make slider hooks to lower it onto the structural members of the lower section to rest on.
Several things are vital.
One is the safe working load of the rope. Ours was a custom made 1" four strand manila with a reverse lay. Usable life was calculated like drilling cable. Drilling cable was calculated in ton miles, that being how many tons had been lifted, and how many miles of the cable had been run through the sheaves. IOW, wear. Wear could be accurately determined either in this rope, or 1 1/8" drilling cable by caliper measurement and visual wear inspection. Rope was not calipered, and just common sense and visible wear was the criterion. Any broken strands made it a boat anchor rope. We used custom lengths of over 9,000' to change antennas on a 3,000' high KATV TV antenna located south of Kaplan Louisiana were made and used. Remember, you have to go up, and down, and have a 3,000 tagline to hold back the load. Up and down was 6,000 feet to hook on to the antenna, then you had to pull it back up and hold back on the remaining 3,000 feet because the weight of the
3,000 feet on the other side far outweighed the antenna, and once it got up so high, it would take off on you, having more rope weight on one side, and nothing holding it back on the other. Nearly two miles of rope was its own bag of snakes. Draw cartoon diagrams to plan ahead for hold back tag lines where needed.Powering this was either a drilling drawworks, with maybe thousands of horsepower on a drill rig, or if on a land rig and a light derrick, a Continental fourbanger built on an I beam frame, running through a car transmission, and out through a car rear end, but with brass capstan catheads instead of car rims. This was always deadmanned to the ground, or had 12" x 1" flatbar outriggers that pickup trucks would park on with their tires on the flatbar. A cathead is a brass hourglass polished curved round roller that allow for slippage, and the slippage can be controlled by putting a specific number of turns on the cathead, and using hand pressure on the loose end to pull it and make it grab, or loosening and making it slip. Up/down. If you do not have access to a cathead, I have seen them done off a truck rear wheel with a cathead attached. A very long capacity electric winch could also be used, and with a 25 or 50' tower, not a real lot of cable would be needed. By figuring your lengths, two cables could be put together using thimbaled eyes and Crosby clamps, but they would have to be calculated so they would not have to pass through a fairlead or onto a winch drum.
Creating a lifting point directly over the section to be lifted is critical. It is good to have the lifting line come up through the center of the structure, but not absolutely a necessity. Having it straight up rather than at an angle prevents side loading of the gin pole, that is, pulling on it from the side rather than from right under it. You may have to rig a snatch block at the bottom of the tower to fairlead the rope/cable through, and change directions sending it up top. It may be able to do by parking the cathead close to the base, just don't get it more than about ten feet away from the base, as you will be pulling too far sideways, possibly creating a catastrophic spectacular youtube failure.
Preparation is vital. You can loosen up the bolts and nuts on the section you want to remove prior to removal, and this will tell you if they are going to come off, will need some extra torque, are going to require an air impact, or in worst case scenario, the need of a smoke wrench. (cutting torch) But if you have your gin pole in place, secure, and lines taut, the cutting of the bolts should not create a shock load on the section when it does come loose. Watch the slag so it does not burn the rope. Avoid shock loading at all costs, as momentary loads come into play, and can exceed SWL.
All this has to be a ballet. Slow and precise. No herky jerky. If you have made your gin pole in a good design, it will support probably 5 times what you are trying to lift. Buy quality steel, quality sheave, quality chain, and quality welding. Use specific shackles to fasten the chain, and NOT grab hooks. Build for weight and strength, but still try to make it as light as you can, and it must be at least 60% of the length of the segment to be removed/replaced. Tag lines and follow lines wrapped around the in place segment can keep it from getting too far out of hand as it comes down or goes up, just watch for hanging up on bolt heads or anything else. Cable or chain can also be used. You want the slider cable/chain at the bottom of the sliding section to keep it from flipping. Slow is the rule of the day.
Once you separate a section from the bolts, it will be hanging vertically over your lower stack. Just push it aside, and let your cathead operator lower it, scuffing along the tower, or attach a tag line, and have a ground hand pull it away from the stack to prevent scuffing or hanging up. If you want it to scuff along the in place sections, put your safety loop around it. Do not use a vehicle, only manpower, as a vehicle can apply an excessive amount of sideloading.
Now, your cathead and operator. It is a dirty oilfield heavy metal operation that requires the touch of a surgeon. It takes ONE cathead operator, and ONE person giving directions, so have a meeting ahead of time, and assign a signal man, and instruct everyone else to either keep their hands in their pockets or stuffed in their belt behind their backs, and their mouths shut unless they see something clearly dangerous.
All gin poles must be manufactured with different things in mind. First, the configuration of the element to be hoisted or lowered, which will determine the configuration and shape of the "shoe" on which the pole will rest. Second is the weight of the section that is going to be lifted or lowered, wanting the gin pole to be at least 60% of the length of the section.. Next is the length, although as long as you attach the lifting line past center point vertically, you can use a shorter gin pole than the section you are hoisting/lowering.
Gin pole configuration can consist of triangulated arrangement of pipes, and then a continuous W shape of reinforcements, similar to what is used in bar joists. Attachment of top pulley and bottom shoe is critical.
A consideration is weight, knowing that you will have to hoist this gin pole in place to get to the highest section. In my latter days in derrick erection, aluminum poles were coming into fashion, with aluminum welding techniques being improved, and aluminum quality improving in the early 70's. They were also much easier for a man or two to move from one leg to the other, walking on top of the MicroLam scaffold boards.
The bottom section will not be able to be removed with a gin pole. You will have to use tag lines, perhaps attached to trucks to lean it over. Removal of the bottom bolts may be by wrenches, or a cutting torch.
Get yourself a good harness and tie off. Nuff said about that.
I know this is a lot, but in actuality, it is quite simple when eaten one bite at a time. It is doable, but for someone doing it the first time, to pull all of the components together is a bigger deal than a rig builder who will have all this on one trailer. It really ain't rocket surgery, and we have done some massive lifts just using 1" rope and a gin pole.
Trying to find some pics of a gin pole, but no luck so far. If you're interested, I can draw one, and take a pic, and put it on flickr.
Rigbuilding, diving, working on boats, high climbing, working with explosives, oil well drilling, ............. it was like getting to be a pirate.
Steve